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Why some volcanic eruptions are more dangerous than others

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A plane flies over the Bardarbunga volcano as it spews lava and smoke in southeast Iceland on Sept. 14. The Bardarbunga volcano system has been rocked by hundreds of tremors a day since mid-August, prompting fears the volcano could explode.

The recent eruption at Mount Ontake in Japan killed more than three dozen people but in Hawaii, the slow moving lava is being watched by residents just miles away.

While palm trees and cool breezes make Hawaii a vacation favorite, this slice of paradise sits on active volcanoes.

Right now, a slow lava flow is making its way to the town of Pahoa, where hundreds of residents and local businesses are keeping an eye on it from just a couple miles away.

Hawaii isn't the only place where volcanoes are erupting. Similar activity is happening in Iceland, Indonesia, Ecuador, Mexico and, of course, there was the recent eruption at Mount Ontake in Japan which killed more than three dozen people.

Some volcanic eruptions are more dangerous than others, says Dr. Rosaly Lopes, senior research scientist at NASA's Jet Propulsion Laboratory and an expert on volcanoes.

Molten rock, or magma, comes under pressure, but it's how that pressure is released that determines whether the volcano will create a violent, sudden eruption or not, she says.

"If the magma is very sticky, very viscus, it will explode. If the magma is not very sticky, the magma can come out slowly and then the volcano is not explosive," says Lopes. "So it really is a matter of the amount of gas in the magma and how easily it can come out."

Lopes, who also studies ice volcanoes on Titan, Saturn's largest moon, says there's lots of interesting and lesser-known facts about how volcanoes work.

“It’s actually possible to walk on a moving flow because the crust at the top is a very good insulator and it will protect you," she said. Though temperatures can get up to 1,200 degrees Celsius, the crust cools quickly.

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